Unusual effects on hydrogenation: anomalous expansion and volume contraction

The present paper considers two unique volume effects associated with the formation of intermetallic hydrides, anomalously high linear and volume expansion (CeNi3→CeNi3D2.8) and unusual volume contraction (HoNiSn→HoNiSnD0.67). The crystal structures of both deuterides were solved on the basis of powder neutron diffraction data. The hexagonal CeNi3 anisotropically expands on hydrogenation along 00z (30.7%). In orthorhombic CeNi3D2.8 (space group Pmcn (No. 62); a=4.8748(3); b=8.5590(5); c=21.590(2) A) this expansion proceeds within the CeNi2 slabs only (63.1%), with a 'shrinking' of the CeNi5 parts (-2.8%). All D atoms are located inside the CeNi2 part and on the border of CeNi2 and CeNi5. In sharp contrast to the known crystal structures of intermetallic hydrides, in CeNi3D2.8 deuterium atoms do not fill initially existing interstices but, instead, attract cerium atoms into their surrounding and form new D-occupied sites, Ce3Ni and Ce3Ni3. The hydrogenation of HoNiSn causes a transition from TiNiSi to the ZrNiAl type of structure. Transformation of the metallic sublattice into the ZrNiAl type results in a 'shrinking' of the Ho3Ni tetrahedra filled by D. Their occupation in HoNiSnD0.67 (space group P6-bar 2m (No. 189); a=7.24197(10); c=3.93514(7) A) proceeds with a formal 'negative' volume expansion effect and leads to the formation of strong Ho-H bonds